Chemical mechanical polishing (CMP) is presently used to polish a variety of materials found in semiconductor devices. Those materials include metals, such as tungsten, aluminum, and copper. Regardless of the type of material being polished, similar techniques are used. For example, a polishing system typically includes a polishing platen, on which is attached a polishing pad. While the platen is being rotated, a slurry is dispensed while a semiconductor wafer is pressed against the pad. A combination of the chemical reaction between the slurry and the layer being polished and the mechanical interaction between abrasives within the slurry and the layer being polished cause the planarization of the layer.
One factor that affects the characteristics of a polishing process is the type of polishing pad used. FIG. 1 illustrates a pad 10 containing a plurality of fibers 12 interspersed within a polyurethane matrix. In commercially available pads, fibers 12 include polyester or cellulose. One such commercially available polishing pad is the Suba 500 pad sold by Rodel, Inc. of Wilmington, Del., which has polyester fibers. FIG. 2 illustrates a polishing pad 14 that includes a plurality of polymer particles 16 and a plurality of voids 17. Voids 17 are created in polyurethane matrix 18 as a result of a heating process. A commercially available polishing pad having a structure similar to that illustrated in FIG. 2 is the IC-1000 pad also manufactured and sold by Rodel, Inc.
Polishing pads, such as those illustrated in FIGS. 1 and 2, do not provide ideal conditions for polishing two dissimilar materials during the same polishing operation. For example, when polishing a conductive layer that overlies an oxide layer, the conductive material is likely to be removed faster around the perimeter of the wafer compared to the center. Consequently, the polishing pad is exposed to an oxide layer and a conductive layer simultaneously. One problem is that a phenomenon known as glazing occurs and causes the pad surface to become smooth. To combat the problem of glazing, conditioning is performed using a diamond disk, for example. Conditioning is a process whereby the polishing pad is restored to close to its original porosity and texture by grinding away a very thin layer off the surface of the polishing pad. A diamond disk is used to accomplish this removal due to its hardness.
The hard conditioning disks impose a problem, particularly used in conjunction with polishing conductive materials. Commercially available diamond disks include diamond particles that are held in place on a disk by a plated metal, such as nickel. If conditioning occurs while a conductive layer is being polished, the slurry, which is used to remove the conductive layer, typically attacks the plating metal used to hold the diamonds on the conditioning disk. Consequently, over time the diamond particles on the disk loosen and contaminate the slurry and can lead to scratches and high particle counts on a wafer, among other problems.
Apart from the problems of polishing conductive and non-conductive (oxide) materials during a same polishing step, there are also problems in polishing two different conductive materials in the same step. For example, when polishing tungsten that is deposited on a titanium/titanium nitride layer, the polishing properties of tungsten and the titanium materials differ greatly. Titanium and titanium nitride are relatively difficult materials to polish using a process optimized for tungsten polishing. Slurry formulations that successfully polish titanium and titanium nitride do not polish tungsten as fast as other slurries. Yet, these other slurries are inefficient in removing the titanium or titanium nitride. In most cases, optimizing the polishing conditions for one material, for example tungsten, leads to a degradation of the polishing characteristics of the other materials, such as titanium or titanium nitride.
According, there is a need in the industry to establish a polishing process that effectively can polish two dissimilar materials in a cost effective manner that is conducive to a manufacturing environment.